This invention pertains to a novel process for preparing 3-methyltetrahydrofuran (MeTHF) from 3-(hydroxymethyl)tetrahydrofuran (HOMeTHF) or 3-formyltetrahydrofuran (3-formyl-THF or FTHF). More specifically, this invention pertains to a process for converting HOMeTHF or FTHF to MeTHF by contacting HOMeTHF or FTHF with a secondary alcohol in the presence of a zirconia catalyst.
The Meerwein-Ponndorf-Verley reaction is a mild and selective method to reduce aldehydes and ketones using a secondary alcohol as a hydrogen source. The catalyst employed usually is a monomeric secondary trialkoxy aluminum acting formally as an agent to transfer hydride from the alpha-position of the source secondary alcohol to the carbonyl carbon of the target aldehyde or ketone. Other catalysts such as magnesium alkoxides, halo magnesium alkoxides, sodium alkoxides, stannic alkoxides, and zirconium alkoxides also are effective but generally not as effective as the aluminum alkoxides. See, for example, A. L. Wilds, Organic Reactions, 2, 178 (1944). U.S. Pat. No. 4,783,559 disclose the use of partially dehydrated hydroxides of titanium, tin, iron, aluminum, cerium, niobium, and zirconium to catalyze this reaction acting in either the gas or liquid phase in fixed beds as heterogeneous catalysts thereby eliminating the cumbersome workup necessary with the conventional aluminum alkoxides. U.S. Pat. No. 4,810,825 describes the use of these catalysts to reduce carboxylic acids, esters, amides, and nitrites to alcohols using secondary alcohols as the hydrogen source and U.S. Pat. No. 4,847,424 discloses catalysts to reduce carboxylic acids, esters, amides, and nitrites to aldehydes using formic acid as the hydride source. These discoveries have increased the understanding and appreciation of the strength of the hydride transfer potential of these agents since reducing these functional groups normally requires very forcing conditions with special catalysts, as described in Catalytic Hydrogenation Over Platinum Metals, P. N. Rylander, Academic Press, Inc. Pubs, New York (1967) pages 229-237, or stoichiometric amounts of strong metal hydrides as described by W. G. Brown, Organic Reactions, 6, 469 (1951) and in Reagents for Organic Synthesis, L. F. Fieser and M. Fieser, John Wiley and Sons, Inc. Pubs., New York (1967) pages 581-595.
The selective hydrogenolysis of primary alcohols is very difficult unless the alcohol is allylic or benzylic, further suggesting a carbonium ion intermediate. Indeed, alcohols often are used as solvents for the hydrogenolysis of esters and carboxylic acids, as is disclosed in Catalytic Hydrogenation Over Platinum Metals, supra. One reliable method of hydrogenolyzing a primary alcohol is to convert it to an active leaving group derivative such as a tosylate, a mesylate, a halide, or a trifluoroacetate, which then is treated with a strong metal hydride in stoichiometric amounts. However, the cost of the reagents and the inefficient use of materials makes this method impractical for most industrial applications.
While there are numerous literature sources describing routes to MeTHF, most of them use multi-step approaches such as, for example, EP 0 727 422, which employs a four-step process starting with methyl methacrylate. Other processes use expensive starting materials such as citraconic anhydride derived from citric acid.
The MeTHF produced in accordance with the present invention is useful as an industrial solvent and, more importantly, as a monomer in the manufacture of polymers such as elastomers. MeTHF is used extensively as a modifier for plasticizers giving modified glass transition temperatures and broader elastic ranges.
A process has been developed for the conversion of HOMeTHF or FTHF to MeTHF by contacting HOMeTHF or FTHF with a secondary alcohol. The process is performed in the presence of a catalyst that comprises a hydrous zirconia. The process may be run under inert atmosphere or in the presence of hydrogen or a mixture thereof.